Blind velocities exist in coherent pulsed Doppler radars at velocities where the target is not detected due to the fact that its Doppler return frequency occurs at a point in the spectrum where, for example, rejection filters exist in the radar processor. For example, a crossing target has no Doppler component in its return signal, that is, it looks like a stationary target. In this case, the return frequency would be at the transmitter spillover frequency, where there generally exists a filter. Multiples of these blind velocities occur at the PRF frequency multiples, following the Doppler principle that Doppler frequency, f.sub.d, is: EQU f.sub.d =2V/.lambda.
where:
V equals closing or opening velocity between radar and target or clutter, and PA1 .lambda. equals wavelength of carrier frequency.
Thus, for example, a 1 kilohertz PRF radar, operating at X band will have blind velocities at approximately every 15 to 20 miles per hour of closing velocity. If a target were in a crossing pattern, or were incoming or outgoing at one of the blind velocities, it would not be detected by the radar system. It is known in the prior art to change the pulse repetition frequency during the operation of the radar in order to change the blind velocities. However, changing the PRF does not alleviate the masking of crossing targets. For example U.S. Pat. Nos. numbered 3,480,953; 3,858,208; and 3,491,360 are representative of such prior art. FIG. 1 is illustrative of the problem.
It is also well known in the art to tune the local oscillator of a radar receiver in order to track varying Doppler frequencies which result from changes in velocity of a tracked target. Furthermore, it is well known in the art to change the local oscillator frequency of any superheterodyne receiver in order to tune the receiver to a particular incoming frequency, such as during the search process. In the first case, the difference frequency between the local oscillator frequency and the incoming frequency is always the same, that is, the intermediate frequency (IF) of the systems. In the second case, that is where the local oscillator frequency is changed in order to search for the target, the difference between the incoming frequency signal and the local oscillator is linearly a function of the target Doppler frequency.